Microsystems (microelectromechanical systems, MEMS) have made strong gains in importance in recent years. MEMS sensors such as, e.g., MEMS-based inertial sensors are used in countless electronic devices and systems.
The use of secret keys that cannot be read out or copied are the basis for data security in the networked world. These keys are used, e.g., for encrypting data transmissions or also for authenticating network subscribers. After initially pure software keys were used and are still used in non-volatile memories, possibly in especially protected memory areas, the trend is clearly in the direction of hardware-based keys. The currently most prevalent method is the so-called SRAM-PUF (physical unclonable function) in which manufacturing tolerances in transistors result in a random, but repeatable switched state of memory cells after the operating voltage has been applied. SRAM-PUFs, however, have already been read out as well as copied, albeit with considerable effort.
U.S. Patent Application Publication No. 2015/200775A describes the use of MEMS sensors for hardware-based keys. For this purpose, the most varied and unique manufacturing-based electromechanical properties of these sensors such as, e.g., resonant frequencies (modes), capacitances, internal auxiliary signals are captured by an evaluation circuit and combined to form a key. Invasive methods, i.e., opening the sensors, normally result in the destruction of the key since, e.g., die pressure conditions in the interior of the sensor and the mechanical stresses in the sensor change.
Using currently emerging modern infrared measuring methods such as IR vibrometry or IR interferometry, there is the risk that parts of the key may be read out in a non-invasive manner through the silicon that is transparent for IR light, which reduces the security of the key markedly.